7.1
EDITED BY MARINA MD ARSHAD, CSC FSKSM UTM JB
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

7.2
Figure 7.1 Transmission medium and physical layer
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Transmission media are actually located below the physical layer and are directly controlled by the physical layer.
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A transmission medium can be broadly defined as anything that carry information from a source to a destination.

7.3
Figure 7.2 Classes of transmission media

7-1 GUIDED MEDIA
Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable that use metallic (copper) conductors, and fiber-optic cable that accepts and transports signals in the form of light.
Topics discussed in this section:
Twisted-Pair Cable
Coaxial Cable
Fiber-Optic Cable
7.4

Figure 7.3 Twisted-pair cable
7.5
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Consists of two conductors (normally copper), each with its own plastic insulation, twisted together.
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One of the wires is used to carry signals to the receiver, and the other is used only as a ground reference. The receiver uses the difference between the two.
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Interference (noise) and crosstalk may affect both wires and create unwanted signals.

7.6
Figure 7.4 UTP and STP cables

7.7
Table 7.1 Categories of unshielded twisted-pair cables

7.8
Figure 7.5 UTP connector

7.9
Figure 7.6 UTP performance

7.10
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Twisted-pair cables are used in telephone lines to provide voice and data channels.
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The DSL lines that are used by the telephone companies to privide high-datarate connections also use the highbandwidth capability of UTP cables.
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LAN such as 10Base-T and 100Base-T, also use twisted-pair cables.

Figure 7.7 Coaxial cable
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Coax carries signals of higher frequency ranges than twistedpair cable.
Outer metallic wrapping serves both as a shield against noise and as the second conductor that completes the circuit.
7.11

7.12
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Categorized by radio government (RG) ratings.
Table 7.2 Categories of coaxial cables

Figure 7.8 BNC connectors
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Used to connect coax with devices.
Most common: bayone-Neill-Concelman (BNC)
BNC connector is used to connect end of cable to a device such as TV set.
BNC T is used in Ethernet networks to branch out to a connection to a computer or other device.
BNC terminator is used at the end of cable to prevent the reflection of the signal.
7.13

Figure 7.9 Coaxial cable performance
7.14

FIBER-OPTIC CABLE Figure 7.11 Optical fiber
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A fibre optic cable is made of glass or plastic and transmits signals in the form of light.
7.15

Figure 7.14 Fiber construction
7.16

Figure 7.15 Fiber-optic cable connectors
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Subscriber channel (SC) connector is used for cable TV.
Straight-tip (ST) connector is used for connecting cable to networking devices.
MT-RJ is the same size as RJ45
7.17

Figure 7.16 Optical fiber performance
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7.19
Advantages
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Higher bandwidth
Less signal attenuation
Immunity to electromagnetic interference
Resistance to corrosive materials
Light weight
Greater immunity to tapping.
Disadvantages
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Installation and maintenance.
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Unidirectional light propagation.
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Cost.

7-2 UNGUIDED MEDIA: WIRELESS
Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication.
Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them.
Topics discussed in this section:
Radio Waves
Microwaves
Infrared
7.20

Figure 7.17 Electromagnetic spectrum for wireless communication
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Figure 7.18 Propagation methods
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7.23
Table 7.4 Bands

Figure 7.19 Wireless transmission waves
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RADIO WAVES: Figure 7.20 Omnidirectional antenna
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Frequencies between 3kHz and 1 Ghz.
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When an antenna transmits radio waves, they are propagated in all directions
(omnidirectional).
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It is susceptible to interference by another antenna.
Advantage: can penetrate walls.
Disadvantage: cannot isolate inside or outside communication.
Used for multicast communications, such as radio and television, and paging systems.
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7.26
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Frequencies between 1 and 300 GHz.
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Unidirectional. Advantage: not interfering with another pair of aligned antennas.
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Characteristics:
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Line-of-sight propagation.
Very high-frequency microwaves cannot penetrate walls.
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The bad is relatively wide, almost 299 GHz.
Use of certain portions of the band requires permission from authorities.
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Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs.

Figure 7.21 Unidirectional antennas
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7.28
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Frequencies from 300 GHz to 400 THz.
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Used for short-range communication in a closed area using line-of-sight propagation that prevents interference between systems.  useless for longrange communications.
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Cannot be used outside a building because the sun’s rays contain infrared waves that can interfere with communications.
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Application: IrDA ( with a PC
Infrared Data Association ) port that allows a wireless keyboard to communicate